1. Technical Field
The present invention relates to a color processing technique which converts a target device color signal in a target device in a color space in N (N≧4) dimensions or more including black ink to an output device color signal in an output device in a color space in M (M≧4) dimensions or more including black ink or is used for conversion.
More specifically, the present invention relates to a color processing technique in which when a color outputted from a target device is simulated by an output device, the color is reproduced at high accuracy with satisfactory gradation by the output device on which a color material total quantity is limited while holding the black ink pattern of the target device insofar as possible.
2. Related Art
There is a technique for simulating color reproduction of a target device by a output device using four or more color inks such as CMYK including black ink.
The color reproduction characteristic of the target device and that of the output device are generally different. When attempting to directly use the black ink pattern of the target device in the output device for simulating color reproduction in the target device, the color gamut reproducible by the output device cannot be effectively used. In many cases, quantity of color material used for simulation by a xerography printer or an ink jet printer is limited. The limited quantity increases the difference in the color reproduction characteristic between the target device and the output device.
To realize high precision ink simulation by holding the black ink pattern of the target device, a technique is needed to hold quantity of black ink in the output device.
As such the technique, there is a method described in Japanese Published Unexamined Patent Application No. 2004-112269. In the method described in Japanese Published Unexamined Patent Application No. 2004-112269, a color signal in a device-dependent color space of a target device (hereinafter, called a device color signal) is converted to a color signal in a device-independent color space (hereinafter, called a device-independent color signal), thereby calculating a minimum black ink quantity required to satisfy a color material total quantity limit which can reproduce the device-independent color signal by an output device. A black ink quantity is determined from the minimum black ink quantity required and the black ink quantity of the target device color signal according to chroma. An output device color signal is calculated from the target device-independent color signal and the calculated black ink quantity.
In the method described in Japanese Published Unexamined Patent Application No. 2004-112269, the minimum black ink quantity required is calculated by search in the device-independent color space, resulting in longer processing time. To address such problem, there is a method described in Japanese Published Unexamined Patent Application No. 2005-064774. In Japanese Published Unexamined Patent Application No. 2004-112269, the minimum black ink quantity required to satisfy the color material total quantity limit which can reproduce the target device-independent color signal by the output device is calculated in a search manner. Instead of that, the method of Japanese Published Unexamined Patent Application No. 2005-064774 constructs a model which calculates a pair of any device-independent color signal and a minimum black ink quantity required to calculate the minimum black ink quantity required from the device-independent color signal so that this model is used to calculate the minimum black ink quantity required at high speed.
Here, the pair of the device-independent color signal and the minimum black ink quantity required can be made by a device-independent color signal corresponding to an output device color signal in which any color material except for black ink is 100% or is equal to a color material total quantity limit value and a corresponding black ink quantity.
FIGS. 9A and 9B are explanatory views of the changes of lower side color gamut surfaces in a color gamut when a black ink quantity is changed in a device-independent color space. In FIG. 9A, a device-independent color signal corresponding to an output device color signal in which any color material except for black ink is 100% or is equal to a color material total quantity limit value is indicated by a black circle. The lower side color gamut surface in which the black circles (device-independent color signals) corresponding to each black ink quantity exist is indicated by a solid line. The black ink quantity corresponding to the lower side color gamut surface in which the respective black circles exist is a minimum black ink quantity required corresponding to the device-independent color signal.
According to the above-described related art, utilizing the color gamut of the output device effectively, color reproduction of the target device can be simulated at high accuracy while holding the black ink pattern of the target device insofar as possible.
However, color reverse can occur by increasing the quantity of a black ink, when there occurs a state in which the color material total quantity is limited on a lower side color gamut surface corresponding to a certain black ink quantity and the color material total quantity is not limited on a lower side color gamut surface corresponding to a different black ink quantity. For example, in FIG. 9B, the color material total quantity is not limited on K=0 to 25% and the color material total quantity is limited on K=50 to 100%. In this case, the lower side color gamut surface of K=25% and the lower side color gamut surface of K=50% are intersected with each other. Even when the color material total quantity is limited on all the lower side color gamut surfaces corresponding to respective black ink quantities, the lower side color gamut surfaces can intersect each other depending on the characteristic of the respective color materials used in the output device.
The intersection of the lower side color gamut surfaces means that the device-independent color signals corresponding to an intersecting point allow two or more black ink quantities matched with each other. When a model is created by a device-independent color signal and minimum quantity of black ink to be required, the minimum quantity of the black ink will be different from quantity to be expected. When a target device color signal is converted to an output device color signal based on the black ink quantity different from the expected black ink quantity, the color reproduction accuracy can be lowered or the gradation can be deteriorated.
The phenomenon in which a minimum black ink quantity required is not changed monotonously in a device-independent color space is a problem for the method of utilizing the model predicting a minimum black ink quantity required from a device-independent color signal using a lower side color gamut surface, as described in Japanese Published Unexamined Patent Application No. 2005-064774. The phenomenon is also a problem in the method described in Japanese Published Unexamined Patent Application No. 2004-112269. Dichotomizing search cannot be used when a minimum black ink quantity required is searched or a minimum black ink quantity required searched to continuous device-independent color signals can cause a gap.